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3. Lymphatic System

What is lymph?

Lymph is fluid that surrounds cells in your tissues – the extra ‘juice’ in the body. When you have a shallow cut or pop a pimple, the clear, watery liquid that oozes out is lymph. Much of your lymph comes from fluid that escapes blood vessels. Thus, the composition of lymph is similar to that of blood plasma (which is what would be left of blood if you took out all the blood cells). Like plasma, lymph contains salt ions, proteins, and clotting factors. Lymph can also contain fat absorbed from the food you eat. Since much of your lymph comes from your blood vessels, the body has developed channels called lymphatic vessels that deliver lymph fluid back into the blood, to keep up your blood fluid levels. The lymphatic system is like system of gutters that direct fluid back into the main reservoir.

How does the lymphatic system relate to the immune system?

Since lymph moves amongst your tissues and is channeled throughout the body, lymph can carry bits of invading organisms and other signals of infection from the site of infection to immune system cells. Lymph nodes are encapsulated clusters of immune system cells, usually less than a centimetre wide, that are connected to lymphatic vessels. Lymph drains through lymph nodes. There are about 600 lymph nodes throughout the body. Most of the immune cells in lymph nodes are lymphocytes. Note that some lymphocytes also circulate in lymph itself and some lymphocytes are also found in ‘extranodal lymphatic tissue’ – meaning the lymphocytes aren’t wrapped up in a little package like they are in nodes, but there is still a bunch of lymphocytes clustered in that tissue.

How do lymphocytes develop and act in the immune system?

There are two types of lymphocytes, B-cells and T-cells. B and T cells differentiate from cells in bone marrow and are released into the blood. B-cells in the blood can travel to lymph nodes or extranodal lymphatic tissue, and wait there until they receive signals of infection in the body. This can trigger B-cells to proliferate and differentiate into cells that either produce antibodies (plasma cells) or cells that remain in the body after the infection, to be reactivated later if the same infection occurs again (memory B-cells). In contrast, when immature T-cells are released from the bone marrow to the blood, they move to the thymus – the thymus is a small gland in the upper chest where T-cells mature. The mature T-cells re-enter the blood and move to lymph nodes. In an immune response, T-cells can be recruited to help kill cells that are infected by a virus or cancerous.

What does the spleen do?

The spleen is in some ways like a giant lymph node. It contains many lymphocytes and lymph flows through it, delivering signals of infection to lymphocytes. The spleen is also involved in removing old red blood cells from the circulation.